1. Field of the Invention
The present disclosure is directed toward a multiphase drilling system and one which attains an underbalance in system pressure. More specifically, in drilling an oil well, the most popular approach is drilling the well with a drill bit affixed to the end of a string of drill pipe which is used to pump down drilling mud circulating through the drill bit at the end of the pipe where the mud is returned to the surface on the exterior of the drill pipe flowing upwardly in the annular space on the outside of the drill pipe. The mud is kept at a specific weight, typically measured in pounds per gallon, so the weight of the column of mud in the partially drilled well is equal to and preferably greater than the pressure that would prevail in the formations as the well is drilled to deeper depths.
2. Background of the Art
There is a preexistent pressure on the formations of the earth which, in general, increases as a function of depth due to the weight of the overburden on a particular strata. Intuitively, this weight increases with depth so the prevailing or quiescent bottom hole pressure is increased in a linear fashion with respect to depth. Thus, as the well depth is doubled. the pressure is likewise doubled. There are, however, some formations which have a fluid drive which is at a higher pressure. When a drill string or "drill stem" penetrates such a formation, fluid may flow in the formation toward the open hole and flow into the annular space, thereby venting and changing the mud pressure balance. This is especially true when a formation is entered where there is a relatively high pressure fluid drive and the formation also includes a significant portion of natural gas. The gas may readily flow out of the formation into the well borehole and bubble upwardly. The formation may produce natural gas in such volumes that the standing column of drilling fluid which maintains bottom hole pressure equal to or greater than the pressure at that depth may be significantly reduced. So to speak, the column of drilling mud is foamed and can become so light that a blowout occurs.
Blowouts are a threat to drilling operations, and especially create significant risk to personnel. Since the well borehole may puncture a formation, perhaps at an expected location or perhaps in an unexpected fashion, it is possible for a significant unexpected flow of natural gas to be encountered. In the past, the first warning on the rig floor at the surface has been a threatening reduction in mud weight. That, however, is difficult to visually inspect at the surface. Even worse, in catastrophic circumstances, the first warning at the rig floor is that the gas flow released from the confined formation punctured by the well borehole is sufficient to lift the drill string. In the worst occasion, the drill pipe has actually been blown back out of the partly completed well. The gas cut mud is blown up through the annulus, forced from the well, and gas begins to flow without limit.
Protection has been obtained, with some success but with occasional failures by installing a blowout preventer (BOP hereinafter) at the rig floor. Indeed, safety demands BOP installation and it is mandatory that a BOP is installed. They, however, do not always work in sufficient time to maintain and keep control over a blowout.
One approach used heretofore has been to drill the well using drilling mud which provides an overbalance in pressure at the bottom of the partly complete well borehole. An overbalanced is attained by increasing the density of the drilling fluid. If only water were used, the specific density would be minimal. The weight is increased by adding weight materials which are typically clay products. The density can be raised significantly by adding the weight materials to the drilling mud. That provides a substantial measure of safety because the weight of the mud can be increased so much that overbalancing of the bottom hole pressure is always a prevailing fact.
The column of drilling mud in the annual space is increased in weight until the weight is so high there is no risk. One detrimental aspect to this is, as the weight is increased, the rate of penetration of the drill bit is decreased. The drill bit operates by rotating cutting teeth jammed against the bottom face of the partly completed well borehole. They tend to fracture pieces of the formation then being drilled. The formation, however, is held in place by the column of drilling mud. If the column of mud were omitted, the formation would more readily fracture, and the rate of penetration of the well into the earth would be substantially increased.
Some have attempted to do this by air drilling. Air drilling is a process which involves the circulation of air through the string of drill pipe. Air drill has met with only modest success. It is perhaps most successful in stone quarries and the like. Air is conducted down the string of drill pipe and out through the drill bit. The air is less effective than drilling mud in maintaining bottom hole pressure blunt it enables all increase in the rate of penetration.
One aspect of successful operation in drilling with air is that increased rate of penetration. Cuttings are blown away but they are not carried as readily through the annular space. They are more readily removed by the column of drilling mud which serves a cleaning and scavenging purpose. The column of return mud is intended to carry all cuttings out of the well borehole and that is normally the case. In addition, the drilling mud cools the drill bit which generates substantial heat as a result of the frictional aspect of the drilling process. In part, this has been dealt with by adding water mist to high pressure air pumped into an air drilling rig. There is some cooling from the water. In addition, it tends to wet the dust which is formed by the drilling and enables an improved return rate with some reduction in dust.